The present invention relates to a vacuum evaporation process for applying gold coatings to materials such as quartz and, in particular, to such a process and a resultant layered gold coating characterized by reproducibility, high reflectivity, strong adhesion and resistance to erosion and corrosion.
The need for the present gold coating and associated process arose in the semiconductor fabrication industry and specifically with respect to the recent use of reflective gold coatings which are applied to inductively heated quartz epitaxial growth reactors to enhance performance thereof. The use and advantages of such coatings are described in U.S. Pat. No. 4,579,080, which issued Apr. 1, 1986 as a continuation-in-part of U.S. patent application, Ser. No. 560,085, filed Dec. 9, 1983, in the name of Martin et al, entitled "Induction Heated Reactor System for Chemical Vapor Deposition", now abandoned, which patent is hereby incorporated by reference.
The use of a reflective coating is derived from the need to eliminate or substantially reduce the thermal gradients across semiconductor wafers during thermal processing thereof. Thermal stress in semiconductor wafers during processing can generate dislocations which propagate along the crystallographic lattice in characteristic, frequently visible slip planes.
In the past, thermal stress-induced crystallographic slip has manifested itself in high temperature processing such as the high temperature growth of epitaxial silicon layers which are used, for example, to form bipolar IC components. Stress-induced slip itself, as well as its unwanted function of acting as a sink for impurities, lead to defective transistors which in turn lead to defective circuits on individual IC chips and defective chips, thereby reducing the yield in the semiconductor processing operation.
FIGS. 1 and 2 illustrate one embodiment of the high temperature epitaxial reactor system disclosed in the Martin patent which uses a reflective coating to decrease thermal gradients across the wafers during processing and, thus, substantially eliminate slip. The reactor system 10 includes an inner quartz bell jar 11 and a concentric outer dielectric bell jar 12 which define a water cooling channel 13 therebetween. A water inlet 14 and a water outlet 16 are provided for bringing cooling de-ionized water into the channel 13 and withdrawing the heated water from the channel. A generally cylindrical susceptor 17 is supported on a susceptor lift and rotation assembly 18 for rotating the susceptor 17 during processing of the semiconductor wafers and for moving the susceptor in and out of the inner quartz bell jar 11 for loading and unloading of the wafers 25.
An infrared reflecting coating 20 is formed on the exterior wall surface of the inner quartz bell jar 11. See FIG. 2. During chemical vapor deposition, for example, of epitaxial wafer layers the gaseous chemicals are brought into the interior of the inner bell jar 11 through a gas inlet 23. The gases flow down the sides of the cylindrical susceptor 17 and are exhausted out the bottom through gas exhaust ports 24. Deposition of the epitaxial layer on a substrate is effected by concurrently applying electric current from supply 26 to the induction coil 21, typically at a frequency within the range of about 5 to 15 kilohertz, to induce corresponding heating currents in the susceptor for heating the substrate.
As is discussed- in some length in the Martin patent, without the reactor design disclosed therein, including the reflective coating 20, during high temperature processing, in the range of 1,000.degree. to 1,200.degree. C., backside heating of the wafers 25 by the susceptor can cause substantial thermal gradients through and across the substrate which, as mentioned, result in crystallographic slip.
In the illustrated reactor system 10, the inner wall or bell jar 11 is formed from a material such as quartz which is highly transparent to the infrared radiation emitted by the susceptor 17 and wafers 25. About 75 percent of the infrared energy radiated from the susceptor is transmitted to the quartz wall 11, and about 90 percent of the radiated energy in the 0.5-4 micron band is transmitted through the quartz layer 11 and strike the reflecting coating 20. Thus, if the coating 20 has a suitably high reflectance value for the 0.5-4 micron band of infrared radiation (such as, for example 95 percent efficiency) the 75 percent of the radiated infrared energy is transmitted through the quartz wall 11 with 90 percent efficiency, reflected back by coating 20 with 95 percent efficiency and then retransmitted through the wall 11 with 90 percent efficiency, so that more than 50 percent of the energy radiated from the susceptor 17 and wafers 25 is returned to the radiating surfaces. This substantially reduces the front side wafer heat loss and the temperature gradient between the front and backside of the wafers 25. The result is the previously mentioned substantial reduction in crystallographic slip. In addition, data taken for the reactor 10 indicates that there is a substantial decrease in the power, from 70 kilowatts for a system with no reflective coating to about 50 kilowatts to the system with the reflective coating 20, required to reach a processing temperature of about 1,200.degree. C.
In short, a coating 20 having the required uniform reflectance has the advantage of decreasing power reducing crystallographic slip and, perhaps, enhancing deposition uniformity.
The above reflectance or reflectivity estimates and the associated advantages assume nearly ideal characteristics of reflectivity as well as adhesion and durability. The preferred coating has been gold, applied in the form of widely commercially available, spray-on or painted-on, typically colloidal solutions of gold and other base metals/alloys, collectively termed "liquid gold" herein. This process involves some difficulty in achieving reproducibility and uniformity. In addition, the spray-on/paint-on solutions can be applied in maximum thicknesses of about 1,500 Angstroms. While these relatively thin coatings can provide the desired reflectivity/transmission of 96 percent/4 percent, the coatings have proven to be relatively short-lived in the harsh environment of reactors such as 10. The reasons are several.
First, although the outer surface of the inner quartz bell jar 11 and the sprayed-on/painted-on infrared reflecting coating 20 thereon are cooled by the passage of coolant fluids such as de-ionized water along the channel between the inner quartz bell jar 11 and the outer dielectric bell jar 12, nonetheless the outer wall of the inner jar 11 and the coating 20 are frequently at temperatures of about 100.degree. C. during high temperature processing.
Second, the inside wall of bell jar 11 must be cleaned periodicllly to remove deposits, using acids such as combinations of concentrated nitric acid and hydrofluoric acid, with the risk that the gold coating may be subject to a chemical attack. In addition, the coating is subject to scratching and abrasion during handling.
Finally and perhaps most importantly, the conventional sprayed-on/painted-on colloidal gold coatings are eroded or corroded by the de-ionized water which is used to cool the reactor. The erosion and the degradation of the adhesion, density and reflectivity of the relatively thin gold coating are manifested by peeling and flaking. This decreases the reflectivity and renders the reflectivity non-uniform across the bell jar surface after relatively short periods of use. It is then necessary to remove the bell jar from service in order to remove and replace the gold coating.
In view of the above discussion, it is a primary object of the present invention to provide a gold coating process which provides excellent adhesion to materials such as quartz.
It is a related object to provide a gold coating process which is easy to use, yet provides high reflectivity, durable coatings which can withstand the harsh operating environment of quartz bell jars used in high temperature semiconductor processing.
It is one specific object to provide an infrared reflective coating for quartz which can withstand temperatures of about 200.degree. C. and is resistant to attack by de-ionized water.
The objective of excellent adhesion is achieved by a process which involves vapor depositing at least a layer of gold chromium alloy onto quartz and then heat treating the alloy to adhere the alloy to the quartz.
In another aspect, one which combines excellent adhesion and resistance to attack by de-ionized water, the present invention comprises vacuum depositing at least a layer of gold chromium alloy onto quartz, then applying a polymer protective overcoat and heat treating the resulting laminate to firmly adhere the gold coating to the quartz and to the protective polymer overcoat.
In a presently preferred embodiment, the reflective coating comprises two layers: a relatively thin inner layer of heat-treated gold chromium alloy which provides adhesion to the quartz surface and a thicker, outer reflective layer of substantially pure gold. Preferably this two-layer coating is covered with a protective polymer overcoat.